INDICATION OF A NEAR SURFACE CLOUD LAYER ON VENUS FROM REANALYSIS OF VENERA 13/14 SPECTROPHOTOMETER DATA

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1 INDICATION OF A NEAR SURFACE CLOUD LAYER ON VENUS FROM REANALYSIS OF VENERA 3/4 SPECTROPHOTOMETER DATA B. Greger (), N. I. Ignatev (2), N. M. Hoekzema (), and H. U. Keller () () Max-Planck-Insttut für Aeronome, 379 Katlenburg-Lndau, Germany (2) Space Research Insttute, 7997 Moscow, Russan federaton ABSTRACT Radance measurements by an entry probe durng ts descent through the atmosphere allow to retreve a vertcal profle of the optcal propertes. The retreval problem s n prncple smlar for the Venera probes, the last of whch landed on Venus n 982, and the Huygens probe, whch wll land on Ttan n January 25. However, for the optcally very thck atmosphere of Venus, an approxmaton of the angular dependence of the radance allows an analytcal retreval of the optcal propertes, whle ths s not possble for the optcally less thck atmosphere of Ttan. Therefore the Ttan Inverse Radaton Model (T RM) has been developed, whch numercally computes the radatve transfer and estmates optcal propertes by assmlatng measurements from the Descent Imager/Spectral Radometer of the Huygens probe. Both methods the analytcal approxmaton and a modfed verson of T RM are used to estmate the extncton profle throughout Venus atmosphere from Venera spectrophotometer measurements. We fnd a pronounced layer of ncreased extncton at an alttude of 2 km above the surface ndcated by the data of Venera 3 as well as by the data of Venera 4. Ths can be nterpreted as a cloud deck. It may be related to surface areas of hgh radar reflectvty and low rado emssvty whch can be notced at hgher elevatons n the Magellan Venus orbter data. The materal formng the cloud deck and accumulatng onto the hghlands of Venus could be small sold partcles of PbS (galena) or B 2 S 3 (bsmuthte). Key words: Venus; Venera; Ttan; Huygens.. INTRODUCTION If the optcal propertes throughout a planetary atmosphere and the boundary condtons.e., solar nsolaton at the top and surface albedo at the bottom are known, the radance nsde the atmosphere can be modeled by radatve transfer compu- I (Θ) I (Θ) z Extncton: α Albedo: w Phase functon: P (θ) I (Θ) I (Θ) Fgure. Retrevng the optcal propertes volume extncton coeffcent α, sngle scatterng albedo w, and scatterng phase functon P (θ) n an atmospherc layer between two successve radance measurements I and I taken durng the descent. tatons (see e.g. Yanovtskj 997). When analyzng radance measurements from a descent probe, we have to solve the nverse problem to retreve the optcal propertes of the atmosphere, cf. Fg.. In January 25, the Huygens probe wll descent through the atmosphere of Saturn s moon Ttan and ts Descent Imager/Spectral Radometer (DISR) wll take mages and spectral measurements at varous drectons and wavelengths (Tomasko et al. 997a). Ttan s unque n the solar system as t s the only moon wth a consderable atmosphere (Taylor & Coustens 998). At the surface, the pressure s.5 tmes that of Earth s atmosphere. Photolyss of the atmospherc consttuents ntrogen and methane n the stratosphere leads to a complex organc chemstry. By aggregaton aerosols are produced whch form the orange haze layer observed by the Voyager probes (Rages et al. 983; West et al. 983). At vsble wavelengths, the optcal thckness of Ttan s atmosphere s about three (McKay et al. 2). The sun s stll vsble at the surface, but the total llumnaton s domnated by dffuse radance concentrated n a pronounced solar aureole. To retreve the scatterng phase functon of the aerosols formng the haze layer, the Ttan Inverse Radaton Model (T RM) has been developed (Greger

2 2 I (Θ j ) T RM α P (θ) w I(Θ) I Z and I N T RM-V P (θ) α w I(Θ) Fgure 2. Illustraton of the Ttan Inverse Radaton Model (T RM) and the Venera adapted verson (T RM-V). For detals on nput and output quanttes see text. et al. 23a), cf. Fg. 2. It takes as nput the ntensty I (Θ j ) of the downward radance measured by the Solar Aureole Imager (SA), a subnstrument of the DISR, at two wavelength, 5 and 935 nm. The vewng drectons Θ j at whch the SA provdes ntensty measurements comprse two vertcal strpes of 6 5 pxels each, wth a feld of vew of 6 wdth n azmuth and extendng from 25 to 75 n zenth angle. One such strpe s orentated towards the sun (but slghtly off the sun to avod the drect solar beam) and the other n the drecton opposte to the sun (Tomasko et al. 997a). The volume extncton coeffcent α also needed as nput can be estmated from measurements by another DISR subnstrument, the Upward Lookng Vsble Spectrometer (ULVS). The estmaton of α depends on the angular dstrbuton of downward radance. Ths s not known a pror, but as the complete radance feld I(Θ) s an output of T RM, a consstent soluton can be found by teraton (Greger et al. 23b). By assmlatng DISR measurments n ths way, T RM estmates the vertcal profles of scatterng phase functon P (θ) and sngle scatterng albedo w. Smlar to Huygens/DISR, the spectrophotometers on board of the Venera Venus entry probes took radance measurements at dfferent drectons and wavelengths durng the descent through the atmosphere (Moshkn et al. 983). The orgnal ntenson of the work presented heren was to use Venera spectrophotometer data as a test bed provdng a consstency check for the applcaton of a modfed verson of T RM. However, there are some consderable dfferences between Ttan and Huygens/DISR data on one hand and Venus and Venera spectrophotometer data on the other hand. One dfference s the angular resoluton of measurements. Whle the SA of the DISR wll provde mages of the solar aureole that allow to retreve the phase functon of the scatterng partcles, only measurements n two drectons, zenth and nadr, where avalable from Venera 3 and 4 spectrophotometers. The Venera radance measurements are descrbed n secton 2. Another dfference les n the optcal thckness of the atmospheres of Ttan and Venus. At the two Table. Propertes of the Venera landng stes. Venera 3 Venera 4 Lattude Longtude 35 3 Alttude ± 2 m 6 ± 2 m Temperature 738 K 743 K Pressure 89.5 atm 93.5 atm wavelengths where the SA takes mages, 5 and 935 nm, the drect solar beam penetrates the atmosphere down to the surface (Tomasko et al. 997b) and the dffuse radance s concentrated n a pronounced solar aureole. On the contrary, Venera 3 and 4 started ther measurements at an alttude of about 6 km (Moroz 983), where they were already enterng the man cloud layer. At the onset of observatons, the sun may have been vsble, but soon the optcal depth became very large (Moroz et al. 98; Moroz 98). The drect solar beam was obscured and the angular dependence of the radance approached a qute smple pattern ndependent of azmuth. As a consequence, t s possble to analytcally retreve optcal propertes assumng the lmt case of radance measurements nfntely deep n an optcally very thck atmosphere (Ekonomov et al. 983). The results of ths analytcal nverson are presented n secton 3. The extncton profles obtaned n ths way exhbt an ntrgung layer of ncreased extncton close to the surface. At frst, ths appears spurous, because the analytcal approxmaton rested on the assumpton that maesurements are taken deep n the atmosphere, far away from the upper and the lower boundary. Ths assumpton does not neccessarly hold close to the surface, whch represents the lower boundary of the atmosphere. To verfy the analytcal results, the extncton profle accordng to Venera 4 measurements s also estmated wth an adapted verson of T RM as descrbed n secton 4. Ths more comprehensve approach takes the surface nto account and robustly reproduces the extncton feature. Such a pronounced layer of ncreased extncton close to the surface of Venus has not been reported before. Possble consttuents of a low cloud deck and ts relaton to surface areas of hgh radar reflectvty and low rado emssvty are dscussed n secton VENERA SPECTROPHOTOMETER DATA On March and March 5, 982, Venera 3 and 4, respectvely, reached the surface of Venus as the last, most developed probes of the Venera lander seres, see Fg. 3. Locatons and elevatons of the landng stes are lsted n Tab. (cf. Moroz 983). The landng locatons are mapped onto the surface topography from Magellan radar measurements n Fg. 4. The Magellan orbter data s avalable from the NASA Planetary Data System, e.g. at

3 3 Fgure 3. The Venera 3 lander. The spral structure at the top s the antenna. The plate below t s suffcent to reasonably lmt the descent speed n the dense Venusan atmosphere. No parachute was used n the last phase of the descent. Fgure 4. Venera 3 (upper left dot) and Venera 4 (lower rght dot) landng stes mapped onto the Magellan topography. Both landng stes are close to the zero elevaton solne. The exact elevatons are 33. and m, respectvely. 6 5 There s a dsagreement between the landng alttudes as orgnally estmated and between the elevatons gven by the Magellan topography. However, the uncertantes of the estmated landng locatons as well as the uncertantes of the estmated landng alttudes are probably qute large. Alttude [km] Zenth Nadr Durng the descent of the probes through the atmosphere, the spectrophotometer measured the radance nsde the atmosphere over a wavelength range of 48 4 nm n sx dfferent drectons (Moshkn et al. 983; Moroz 983). The feld of vew of each measurement had a wdth of about 2. About 3 spectra were obtaned by Venera 3 and 4. Unfortunately, new attempts to read the orgnal data from magnetc tapes were unsuccessful, and only graphc materals are stll avalable. The naccessblty of the prme materal s regrettable, but, fortunately, the graphc data are suffcently representatve. Graphs were dgtzed to form a secondary database of satsfactory accuracy (Ignatev et al. 997). However, from the orgnal data for sx vewng drectons, only the data for two of them,.e. one close to zenth and one close to nadr, had been dgtzed so far and were avalable for ths study. The radance measurements for one partcular wavelength are shown n Fgs. 5 and 6. The dfference between Venera 3 and Venera 4 data n the absolute ntenty values les wthng the estmated systematc error of ±4 % (Moshkn et al. 983) Radance [W/m2/mcron/sr] Fgure 5. Verera 3 spectrophotometer radance measurements at 7 7 nm. Alttude [km] Zenth Nadr Radance [W/m2/mcron/sr] geophys.wustl.edu/mgn/data/. Fgure 6. Verera 4 spectrophotometer radance measurements at 7 7 nm.

4 4 3. APPROXIMATIVE ANALYTICAL INVERSION Deep n an optcally thck atmosphere lke Venus, the angular dependence of the radance ntensty approaches a smple pattern. The ntensty I becomes ndependent of the azmuth of the vewng drecton and the dependence on the zenth angle θ can be approxmated by I(θ) = I Z ( b + b cos θ), () wth I Z beng the zenth radance ntensty and b beng some constant (see Ekonomov et al. 983, and references theren). In ths case, volume extncton coeffcent α, sngle scatterng albedo w and phase functon asymmetry factor g are approxmately related to zenth and nadr radances, I Z and I N, respectvely, through α ( g) = α ( w) = I Z I N 3 (I Z I N ) d dz (I Z + I N ), (2) d dz (I Z I N ). (3) These are two equatons for the three unknown parameters α, w, and g, thus an addtonal assumpton has to be made. Smlar to prevous work (Ekonomov et al. 983; Moshkn et al. 983), we assume the phase functon asymmetry factor to be g =.7. Alternatvely, one could thnk of prescrbng the sngle scatterng albedo w and calculatng the extncton α from Eq. (3), but as w s very close to unty n Venus atmosphere, the result for α would depend very senstvely on w. Thus fxng g and usng Eq. (2) to calculate α yelds much more robust results. Of course the phase functon asymmetry factor g s not constant throughout the atmosphere. It vares wth the number densty and sze dstrbuton of aerosol partcles. Thus we can not drectly retreve the extncton α, but rather the product α ( g). To put some ntutve numbers at the graphs shown below, we assume g =.7. Gven a fxed phase functon asymmetry factor, Venera descent probe spectrophotometer measurements of zenth and nadr radance ntenstes can be used to estmate the volume extncton coeffcent and the sngle scatterng albedo through Eqs. (2, 3) from about 6 km alttude down to the surface. However, t s mportant to note that the resultant extnctons are uncertan wthn a factor g. For the estmaton of extncton coeffents accordng to Eq. (2), I Z and I N have to be sampled to the same alttude ponts. Ths type of analyss had bascally been performed before, but only usng a very lmted number of alttude ponts (Ekonomov et al. 983; Moshkn et al. 983). Heren, we supersample the measurements wth a constant vertcal resoluton of m to make full use of the vertcal resoluton of the data that vares between about a hundred meters and a few klometers. Note that ths does not address the Alttude [km] Venera 3 Venera 4 Raylegh scatterng Extncton [/km] Fgure 7. Extncton profles over the complete alttude range as retreved from Verera 3 and 4 spectrophotometer data at 7 7 nm assumng a phase functon asymmetry factor g =.7. Absolute values are uncertan wthn a factor g. orgnal samplng rate of the prmary data durng the descent, but rather the secondary database recovered from graphc materals, cf. secton 2. The respectve analyss of Venera 3 and 4 spectrophotometer data yelds the extncton n dependence on the alttude wth a hgher vertcal resoluton than reported before. The results are presented n Fg. 7. The man cloud deck between 45 and 6 km alttude s clearly vsble n the results from Venera 3 as well as Venera 4. For the later, the double stucture s more pronounced. In Fg. 8, we present a zoom-n of the low alttude range. A pronounced peak of ncreased extncton 2 km above the surface of Venus can be notced. The feature shows up smlarly n the measurements of both Venera probes. In fact, an annomaly n the extncton profle at 2 km alttude shows up over the complete wavelength range of avalable measurements. For Venera 4, ths comprses 48 4 nm, whle for Venera 3 only data for wavelengths longer than 7 nm have been dgtzed approprately so far, cf. secton 2. Fg. 9 shows the optcal depth between and 2 km alttude above the surface as a functon of wavelength. For wavelengths longer than 9 nm, negatv optcal depths show up. Ths s due to the fact that the thermal emsson from the atmosphere becomes sgnfcant at longer wavelengths, and ts contrbuton to the measurements was neglected n the analytcal approxmaton used for the retreval. However, between 7 and 9 nm, the spectra from Venera 3 and Venera 4 agree qute well. These analytcal results obtaned based on Eq. (2) may be questoned close to the bottom of the atmosphere. There the surface becomes vsble (Moroz 22), and the assumpton that the dependence

5 5 Alttude [km] Venera 3 Venera 4 Raylegh scatterng Extncton [/km] Fgure 8. Lke Fg. 7, but a zoom-n of the low alttude range. Optcal thckness Venera 3 Venera Wavelength [mcron] Fgure 9. Retreved optcal thckness between and 2 km alttude. Smlar to the extncton, the absolute values are uncertan wthn a factor of g, cf. Fg. 7. Negatve values are spurous, for detals see text. of the radance ntensty on the zenth angle of the vewng drecton can be approxmated by a the cosne profle as gven by Eq. () does not hold any more for downward lookng drectons. Therefore the nverse estmaton of optcal propertes based on ths assumpton as supposton has to be vewed wth cauton at ths stage. 4. RETRIEVAL OF OPTICAL PROPERTIES WITH T RM To check wether the extncton peak found wth the analytcal approxmaton (secton 3) could be an artefact due to the neglecton of the near surface, we alternatvely estmate the extncton profle wth a slghtly modfed verson of the Ttan Inverse Radaton Model (T RM, Greger et al. 23a,b). The orgnal verson of T RM has been developed to retreve the optcal propertes of the atmosphere of Ttan from measurements by the Descent Imager/Spectral Radometer (DISR) on board the Huygens probe, cf. secton. The DISR wll make smlar observatons as the Venera spectrophotometers albet at hgher spatal and spectral resoluton durng ts descent through Ttan s atmosphere. However, T RM has to be adapted to the more restrcted data stuaton for Venus. The approprately modfed Venera verson T RM-V s llustrated n Fg. 2. Besdes zenth and nadr radance ntensty measurements, I Z and I N, the scatterng phase functon P (θ) has to be provded. It s assumed to be a Henyey Greensten functon wth an asymmetry factor g =.7. Ths asymmetry factor was also assumed n secton 3. The output of T RM-V comprses vertcal profles of volume extncton coeffcent α, sngle scatterng albedo w, and the complete radance ntensty feld I(Θ) n dependence on vewng drecton Θ. Wthn T RM-V, the atmosphere s assumed to be plane-parallel and to consst of 62 evenly spaced layers, wth constant optcal propertes throughout each layer. The altdude range consdered s 62 km. Gven an llumnaton from above and a surface albedo at the bottom of the model atmosphere, the radance s teratvely propagated downward and upward untl the algorthm converges to a consstent soluton. The detals of the radatve transfer computatons are descrbed by Greger et al. (23a). For the Ttan applcaton, the model atmosphere reaches up to 45 km and thus represents the complete alttude range that contrbutes to scatterng processes. As usual n planetary radatve transfer computatons, T RM s llumnated from above by a pont lght source. For the Venus verson T RM-V, the llumnaton of the model atmosphere s dfferent. The Venera probes started ther measurments already deep n the atmosphere and provded only data from about 62 km alttude down to the surface. The model atmophere of T RM-V covers just ths alttude range and thus only represents the lower part

6 6 Alttude [km] Analytcal approxmaton Model, surface albedo. Model, surface albedo.26 A by-product of a T RM-V computaton s the complete radance ntensty I(Θ) for every vewng drecton Θ (cf. Fg. 2),.e., the spatal pattern of sky brghtness. As Venera 3 spectrophotometer measurements are avalable over the complete optcal wavelength range, the sky radance can be computed for any arbtrary wavelength, e.g., for the three colors red, green, and blue. Mergng these three radance ntensty patterns yelds real color mages of the Venusan sky as seen from dfferent alttudes DISCUSSION Extncton [/km] Fgure. Extncton profles retreved from Verera 4 spectrophotometer data at 7-7 nm. by two methods, analytcal (cf. secton 3) and wth a numercal radatve transfer model (cf. secton 4). of the real atmosphere, wth another optcally thck part on top of t. Therefore, we here only llumnate the top of the model atmosphere wth dffuse downward radaton accordng to Eq. (). The parameters of ths cosne profle are chosen to match the frst zenth and nadr measurements taken by the Venera spectrophotometers at about 62 km alttude. Whle teratng downward and upward through the model atmosphere, the extncton coeffcent and sngle scatterng albedo n each layer s contnuously adjusted to reproduce the observed zenth and nadr radance ntenstes. As the scatterng phase functon s prescrbed, there are two parameters n each model layer whch are adjusted to match two observatons at each layer boundary. Hence, a unque soluton s found that almost exactly matches the radance measurements. Radatve transfer computatons have been performed wth assmlaton of Venera 4 zenth and nadr spectrophotometer measurements at 7 7 nm for varous assumed surface albedos. The resultant extncton profles for two dfferent albedos are compared to the analytcal approxmaton n Fg.. The albedo of. represents the value usually assumed for Venus surface (Moroz 983, 22), whle wth.26 the best ft to the nadr lookng measurements close to the surface was obtaned. The extncton profles estmated wth T RM-V dffer only margnally from the analytcal approxmaton. In partcular, there s only a weak dependence on the assumed surface albedo, and the T RM-V results clearly show the same pronounced layer of ncreased extncton 2 km above the surface. Ths supports the ndcaton of a near surface cloud layer, at least t rules out that the fndngs of secton 3 are an artefact of the neglecton of the surface. We have used zenth and nadr radance ntensty measurements to retreve the extncton profle throughout Venus atmosphere. The analytcal approxmaton (secton 3) as well as numercal T RM-V computatons that consder dfferent surface albedos (secton 4) both gve smlar results: A layer wth hgh extncton shows up at 2 km alttude. It s present at all wavelengths between 48 and 4 nm and shows up n the data of both, Venera 3 as well as Venera 4. An mportant queston s wether ths extncton peak observed at the same relatve alttude above the surface by both Venera probes s also located at the same absolute alttude. The alttudes of the Venera 3 and 4 landng stes were estmated to dffer about 7 m, cf. Tab., but mapped on the Magellan topography, they appear qute smlar, see Fg. 4. However, the uncertantes do not allow to rule out one of these possbltes. If we dscard the alttude estmatons and assume the landng locatons to be exact, the extncton peak occurs at constant absolute alttude. Ths s what a cloud deck would look lke. But f the alttude estmatons are correct, the absolute alttude of the extncton peak also dffers about 7 m between the two Venera landng locatons. It should be mentoned that there s some sceptcsm about the valdty of the Venera spectrophotometer data close to the surface, especally for the downward lookng drectons (Moshkn et al. 983; Moroz 22). However, the fact that the extncton feature shows up n a very smlar way for both Venera probes and that t s present over the complete wavelength range makes t dffcult to explan t as an artefact due to an ntrument falure. Therefore, t may be worthwhle to look for processes that could create such a near surface cloud deck as ndcated by the observatons. Venus has been extensvely mapped by radar, ntally from Earth, and later n great detal from Venus orbt by the Magellan spacecraft. Ever snce the 97s scentsts have puzzled over the pecular hgh radar albedo of parts of the planet. Whereas the Venusan lowlands generally have a rather low radar 2 Color mages can not be reproduced wthn ths volume. However, a presentaton comprsng such smulated color mages of the Venusan sky s avalable onlne at

7 7 Varous models have been proposed to explan the radar snow, not all of them allow for any obvous relaton wth the deck of low clouds that we suspect. E.g., a large cover factor of decmeter szed vods n surface rocks can nvoke a hgh radar albedo (Pettengll & Ford 993), or a low-loss sol layer (Tryka & Muhleman 992). However, Magellan bstatc radar observatons of the Maxwell Montes hghlands (Pettengll et al. 996) are best explaned by models n whch the hghlands are covered by a sem-conductng layer (Brackett et al. 995). Varous studes have analyzed chemcal compounds that could make up such a layer. Pyrte (FeS), and other compounds of ron, have propertes that are compatble wth the radar observatons (Pettengll et al. 982; Ford & Pettengll 983), but Fegley (997) states that any pyrte on the surface of Venus should rapdly decompose nto ron-oxdes and sulfur vapor. I.e., the atmospherc abundance of sulfur s too low and the atmosphere s too oxdzng for pyrte to be stable. Fgure. A scatter plot of rado emssvty versus elevaton from the Magellan data tle extendng from 285 to 33 n longtude and from 4 to n lattude. albedo that s compatble wth e.g., dry basaltc rock, the hghlands above alttudes of km (Klose et al. 997) usually show radar albedos as large as.5 or even larger (Pettengll et al. 992b). Such albedos are comparable wth that of snow n the vsble, hence the name radar snow. The same mountanous areas exhbtng hgh radar albedo do also dsplay low radothermal emssvty, as found by Poneer Venus and confrmed by Magellan (Ford & Pettengll 983; Pettengll et al. 992a). Although the Magellan reflectvty data have a far superor spatal resoluton, studes of the anomalous areas have tended to focus on the emssvty data, because they are less strongly affected by surface roughness (Wood 997). The effects of surface roughness can not be subtracted from the reflectvty data wth confdence. It s temptng to presume that there s some relaton between the clouds that we may have observed between and 2 km alttude and ths so-called radar snow that caps the mountans of Venus. Although the radar snow usaually occurs at slghtly hgher alttudes, there s also some ndcaton of t just at the alttude of the extncton feature n the Venera data. In Fg., a scatter plot showng the rado emssvty versus surface elevaton for the 45 4 Magellan data tle comprsng the Venera landng stes s shown. It ndcates preferentally low radar emssvtes at elevatons of 5 8 m. Other scentsts suggested that the radar snow s some sort of volatle sem-metallc compound that s cold-trapped on the coolest locatons of the surface,.e., the hghlands. E.g., Pettengll et al. (996) suggested that the metallc frost mght be elemental tellurum. Ths element would also make an nterestng canddate for cloud formaton. If the compound can form frosts above a gven alttude, t can probably condense nto clouds at slghtly lower alttudes. However, Schaefer & Fegley (23) make a strong pont that elemental tellurum can not exst near the surface snce the element wll be trapped n compounds wth sulfur. Schaefer & Fegley (23) explore the volatle metal chemstry on Venus from the lowest elevatons ( 2.6 km n Dana Chasma) to the bottom of the global man cloud layer at about 5 km alttude usng chemcal equlbrum calculatons that nclude S, Pb, B, Te, Sb and 2 other elements. They consdered about 66 compounds of the trace metals and fnd a few partcularly nterestng canddates for metallc snow: PbS (galena) and B 2 S 3 (bsmuthte). Gven the proper abundances for lead or bsmuth, these could accumulate n the hghlands. Moreover, the compounds have the proper delectrc constants to form radar snow. Mxed compounds of bsmuth and lead wth sulfur may also be good canddates, but at the moment ther propertes are not known well enough to judge. For Earth-lke abundances of bsmuth and lead, Schaefer & Fegley (23) predct that bsmuthte s stable above an alttude of.6 km, and that galena s stable everywhere on Venus surface. If these abundances are smaller by a factor of about 2 for bsmuth, and of about for lead, then both galena and bsmuthte can form and survve at alttudes above 2 3 km. Schaefer & Fegley (23) make a pont that t s reasonable for Venus to have a lower content of B and Pb than Earth. If ether bsmuthte or galena forms from trace amounts of Pb or B n the atmosphere, and f the compound s ndeed stable on the surface of Venus n the hghlands but not n the lowlands, and f t forms the radar snow, then one would expect some sort of transton regon between the lower and the hgher alttudes. I.e., a layer n the atmosphere where par-

8 8 tcles of ths compound form and drft around untl they snow out onto the hghlands or untl they descent nto the lower atmosphere and dsntegrate;.e., a cloud layer near the alttude where the Venera probes may have observed clouds. It seems very worthwhle to model such cloud formaton and to check whether such clouds, and n partcular ther predcted optcal depth as a functon of wavelength, are compatble wth the Venera observatons. ACKNOWLEDGMENTS Ths work was supported by the German Aerospace Center (DLR), contract number 5 OH The Magellan orbter data has been made avalable onlne by the NASA Planetary Data System. REFERENCES Brackett R.A., Fegley B. Jr., Arvdson R.E., 995, J. Geophys. Res.,, 554 Ekonomov A.P., Golovn Y.M., Moroz V.I., Moshkn B.Y., 983, In: Hunten D.M., Coln L., Donahue T.M., Moroz V.I. (eds.) Venus, Space Scence Seres, chap. 9, , The Unversty of Arzona Press, Tucson, Arzona, USA Fegley B., 997, Icarus, 28, 474 Ford P.G., Pettengll G.H., 983, Scence, 22, 379 Greger B., Lemmon M.T., Markewcz W.J., Keller H.U., 23a, Planet. Space Sc., 5, 47 Greger B., Rodn A.V., Salnas S.V., Keller H.U., 23b, Planet. Space Sc., n press Ignatev N.I., Moroz V.I., Moshkn B.E., et al., 997, Planet. Space Sc., 45, 427 Klose K.B., Wood J.A., Hashmoto A., 997, J. Geophys. Res., 97, 6,353 McKay C.P., Coustens A., Samuelson R.E., et al., 2, Planet. Space Sc., 49, 79 Moroz V.I., 98, Space Scence Revews, 29, 3 Moroz V.I., 983, In: Hunten D.M., Coln L., Donahue T.M., Moroz V.I. (eds.) Venus, Space Scence Seres, chap. 5, 45 68, The Unversty of Arzona Press, Tucson, Arzona, USA Moroz V.I., 22, Planet. Space Sc., 5, 287 Moroz V.I., Golovn Y.M., Ekonomov A.P., et al., 98, Nature, 284, 243 Moshkn B.E., Ekonomov A.P., Moroz V.I., et al., 983, Kosmch. Issled., 2, 77 Pettengll G.H., Ford P.G., 993, EOS Trans. AGU, 74, 89 Pettengll G.H., Ford P.G., Nozette S., 982, Scence, 27, 64 Pettengll G.H., Ford P.G., Wlt R.J., 992a, J. Geophys. Res., 992, 39 Pettengll G.H., Wlt R.J., Ford P.G., 992b, In: Lunar and Planetary Inst., Papers Presented to the Internatonal Colloquum on Venus, no. N n SEE, Pettengll G.H., Ford P.G., Smpson R.A., 996, Scence, 272, 628 Rages K., Pollack J.B., Smth P.H., 983, J. geophys. Res., 88, 872 Schaefer L., Fegley B. Jr., 23, submtted to Icarus Taylor F.W., Coustens A., 998, Planet. Space Sc., 46, 85 Tomasko M.G., Doose L.R., Smth P.H., et al., 997a, In: Wlson A. (ed.) Huygens Scence, Payload and Msson, vol. SP-77, 9 38, ESA Publcatons Dvson, ESTEC, Noordwjk, The Netherlands Tomasko M.G., Lemmon M., Doose L.R., et al., 997b, In: Wlson A. (ed.) Huygens Scence, Payload and Msson, vol. SP-77, , ESA Publcatons Dvson, ESTEC, Noordwjk, The Netherlands Tryka K.A., Muhleman B.O., 992, J. Geophys. Res., 97, 3379 West R.A., Lane A.L., Hart H., et al., 983, J. geophys. Res., 88, 8699 Wood J.A., 997, In: Bougher S.W., Hunten D.M., Phllps R.J. (eds.) Venus II Geology, Geophyscs, Atmosphere, and Solar Wnd Envronment, , The Unversty of Arzona Press, Tucson, Arzona, USA Yanovtskj E.G., 997, Lght Scatterng n Inhomogeneous Atmospheres, Sprnger, Berln